For example, a conventional power circuit for a liquid crystal display circuit has been constituted by a constant-voltage circuit for sending out a constant output voltage independently of variation of a source voltage, and a boosting/dropping circuit for boosting/dropping the output voltage of the constant-voltage circuit to thereby send out a plurality of different value output voltages, thereby supplying these output voltages as driving voltages to a liquid crystal display panel to drive it.
It is however impossible for such a conventional power circuit for a liquid crystal display circuit to maintain the quality of liquid crystal display with a low power consumption over a wide range of the source voltage.
For example, assume a case where the liquid crystal display panel needs voltages of five values 0 V, 1 V, 2 V, 3 V and 4 V. In a case where a liquid crystal driving voltage of 2 V is generated in the constant-voltage circuit and liquid crystal driving voltages of 1 V, 3 V and 4 V are generated with this 2 V liquid crystal driving voltage as a reference by the boosting/dropping circuit, the constant-voltage circuit becomes impossible to generate the 2 V liquid crystal driving voltage when the source voltage becomes lower than 2 V, and as a result the boosting/dropping circuit becomes impossible to generate the above-mentioned liquid crystal driving voltages. Therefore there is a problem that the liquid crystal driving voltages drop correspondingly to the drop of the source voltage so that the contrast of the liquid crystal display deteriorates.
In a case where a liquid crystal driving voltage of 1 V is generated in the constant-voltage circuit and liquid crystal driving voltages of 2 V, 3 V and 4 V are generated by the boosting/dropping circuit, the quality of display of the liquid crystal panel can be ensured till the source voltage drops to 1 V. There is however a problem that the loss of charges due to charging and discharging of capacitors is so large that current consumption becomes large to thereby shorten the life of a battery constituting a power source.